Fire Sprinkler System Design for Commercial Buildings

What You Need to Know

Fire sprinklers save lives and protect property. AS 2118.1 (the Australian Standard for automatic fire sprinkler systems) sets the design rules. The NCC (National Construction Code) tells you when a building needs sprinklers. Most commercial buildings over 3 storeys or 25 m in effective height must have them. Get the hazard class wrong, and the whole system is undersized.

• Commercial buildings over 25 m effective height need sprinklers; lower buildings may also need them depending on class and floor area (NCC 2025, Specification 20)
• Sprinkler systems must comply with AS 2118.1-2017 as the deemed-to-satisfy solution (NCC 2025, Spec 20)
• The hazard class sets the design: Light Hazard for offices, Ordinary Hazard for retail and car parks (AS 2118.1, Section 3)
• Light Hazard design density is 2.25 mm/min over a 72 m² design area (AS 2118.1)
• Ordinary Hazard OH2 design density is 5.0 mm/min over a 144 m² design area (AS 2118.1)
• Water supply must last at least 30 minutes for Light Hazard and 60 minutes for Ordinary Hazard (AS 2118.1)
• Maximum coverage per standard sprinkler head is 12 m² with a maximum spacing of 4.0 m (AS 2118.1)

Take a 3-storey office building with 2,000 m² per floor. The offices are Light Hazard. At 2.25 mm/min over a 72 m² design area, the sprinkler demand is about 2.7 L/s (72 m² × 2.25 mm/min ÷ 60). That sounds small, but add in hose stream allowance and pipe friction losses, and the fire pump typically needs to deliver 10–15 L/s at 200–400 kPa.

The water supply tank for this system holds about 20,000 L for the sprinkler reserve alone. If the building also has hydrants (and most commercial buildings do), a combined tank of 40,000–50,000 L is common. That tank needs a pump room next to it, with space for a diesel and electric fire pump set. Allow at least 15–20 m² for the pump room and plan the tank location early.

If part of the building is retail at ground level (Class 6), that area steps up to Ordinary Hazard OH2. The design density more than doubles. The pipe sizes increase, the pump gets bigger, and the tank grows. Mixed-use buildings need the hydraulic design to account for the highest hazard class served by each zone.

Sprinkler heads in office ceilings sit on drops from the branch piping above. Each head covers up to 12 m² with 4.0 m maximum spacing. The heads typically need 25–300 mm clearance below the ceiling. The fire sprinkler contractor needs to coordinate with the HVAC ductwork, cable trays, and lighting in the ceiling void.